Low density fibre-reinforced cement composition

ABSTRACT

A fibre-reinforced, low density cement system for the control of lost circulation during the drilling of subterranean wells, generally comprising a cement particle mixture having a specific granulometric characterization and alkaline resistant fibres, such as fibres composed of glass having a high zirconia concentration or organic fibres composed of a phenolic polymer or other polymeric system.

The present invention relates to a low-density, fibre-reinforced cementcomposition, and in particular to such a cement composition for use incementing oil or gas wells or the like.

When drilling a well such as an oil or gas well, a drilling fluid, oftencalled “mud”, is circulated through the drill string and well to removedrilled cuttings from the bottom of the borehole. This fluid also servesto balance the pressure of fluids in the underground formations throughwhich the borehole passes. At any given depth in the well, the pressureexerted on the walls of the borehole by the fluid will depend on thehydrostatic pressure which in turn is dependent on the depth of the welland the density of the fluid. Certain underground formations are highlypermeable and if the pressure of fluids in the formation is low, forexample because the formation is not very deep or because of fluiddepletion for any reason, the situation can arise in which thehydrostatic pressure of the drilling fluid is greater than that of theformation and fluid goes from the borehole into the formation. Thissituation is known as lost circulation and is undesirable since it isnecessary to replace the lost fluid, because the drilling fluid in theformation displaces any hydrocarbon-bearing fluids and prevents usefulproduction (although this is not always the case) and because excessivelosses can lead to problems with stuck drill pipe in the borehole. Thelost circulation is commonly mitigated by including in the drillingfluid a fluid lost circulation materials. These commonly comprise solidparticulate materials that bridge the fractures or holes of the boreholewhere fluid is lost. prevents further loss of fluid into the formation.

Sometimes the formations are so porous or unstable that it is notpossible to use conventional lost circulation materials to drillingfluids or cement to prevent losses. In such situations, one approach tocure serious lost circulation problems has been to place an impermeablecement plug in the borehole adjacent the formation in question throughwhich the borehole is re-drilled after the cement has set. Whileconventional cement slurries can often be used to set a cement plug forlost circulation control, certain problems can arise when the formationis too weak to support the hydrostatic pressure of a conventional cementslurry without problems of fracturing, borehole instability and/orfurther losses. In such cases, low density cement slurries might beused, an example of which can be found in WO 01/09056. However, lowdensity cement slurries often do not have sufficient strength to supportthemselves and the borehole while setting takes place.

The addition of fibrous materials to cements for a wide variety ofapplication has been well established. R. F. Zollo (Cement and ConcreteComposites, 19, 1997, p 107-122) presents an overview of the types ofcommercially available fibre-reinforced concrete systems and discussionsof how various systems work. Since that time a number of inventions havebeen developed to improve the performance of these materials.

U.S. Pat. No. 5,649,568 (Jul. 22, 1997, W. C. Allen et al. Union OilCompany of California) discloses the use of micro and macro sized glassfibres in a variety of cements including type-G cement widely used inthe oil field in order to improve the corrosion resistance of cementliners exposed to corrosive environments. Micro-fibres were defined asthose having a diameter of between about 10 microns and 70 microns andan aspect ratio of between 5 and 20 to 1. Macro fibres were defined asthose having a diameter between about 10 and 200 microns and an aspectratio preferably greater than 100. Glass was the preferred material.Usage was in the range of 1.5 to 10% by weight based on the weight ofthe total solids. Preferred compositions also included 5 to 50% silicaflour by weight based on the total weight of solid materials.

Glass fibre-reinforced oil field cement systems have been commerciallyavailable since at least 1999. These systems typically contain less than1% fibre by weight based on the weight of cement and typically are inthe medium density range of around 12 pounds per gallon of cementslurry. These cost effective systems have been used mainly for lostcirculation control applications.

U.S. Pat. No. 5,705,233 (Jan. 6, 1998, F. S. Denes et al. WisconsinAlumni Research Foundation) teaches that the types of glass fibrecomposites described above are sensitive to age and curing under alkaliconditions routinely found in cement. Even composites using alkaliresistant glass fibres become brittle after prolonged storage in thepresence of atmospheric moisture or in liquid water environments. Theythen developed a process to modify the surface of polyolefin fibres toincrease compatibility of this surface and cement. This results inimproved bonding and resulting composite performance. The treated fibresare preferably used at a level of less than 1% by volume or about 0.5%by weight.

This general concept has been adapted for oil field use in lightweightcements, especially foamed cements, by Chatterji et al. in U.S. Pat. No.6,220,354 (Apr. 24, 2001, J. Chatterji et al., Hailiburton). This patentteaches that fibres formed of polyesters, polyamides and glass sufferfrom the disadvantage that they degrade in the presence of hydratedlime. These inventors prefer to use polyolefin fibres treated with asurface active agent capable of rendering the fibre surface hydrophilic.Fibrillated net-shaped fibres appear to be preferred and are used inranges of 0.1 to 0.25 weight percent based on the weight of cement.These systems suffer from low compressive strength, typically less thanabout 1500 psi, and high porosity. This makes them less than idea as ameans to control lost circulation situations.

U.S. Pat. No. 6,060,163 (May 9, 2000, A. Naaman, The Regents of theUniversity of Michigan) discusses the optimisation of fibre geometry formaximum effectiveness.

Thus, while a large number of different fibre reinforced cement systemshave been developed for a number of different applications, currentteaching indicates that polyolefin fibres which have been modified toincrease the hydrophilicity of the surface are strongly preferred,especially for light weight cement systems.

The use of fibres in a low-density cement slurry for lost circulationcontrol is discussed in Elmonein, Zaki and Al-Arda, “Cementing thedeepest 20 inch Casing in Abu Dhabi using a combination of Novel LightWeight Slurry and Fiber” ADIPEC-0940, 9^(th) Abu Dhabi InternationalPetroleum Exhibition and Conference, Abu Dhabi, U.A.E. 15- 18 Oct. 2000.

One object of the invention is to provide a reinforced low-densitycement slurry that can be mixed and pumped under conventional oil fieldconditions yet rapidly develops sufficient structure down

eal the well bore and prevent further fluid losses.

The present invention provides a fibre-reinforced, low density cementsystem for the control of lost circulation during the drilling ofsubterranean wells, generally comprising a cement particle mixturehaving a specific granulometric characterization and alkaline resistantfibres, such as fibres composed of glass having a high zirconiaconcentration or organic fibres composed of a phenolic polymer or otherpolymeric system.

The invention also provides a reinforced cement slurry with lowpermeability, low porosity, low erodability and high strength for thelength of time and under the conditions encountered in lost circulationsituation.

A further aspect of the invention provides a continuous fibre networkwhich is used to confine and additionally stabilize cement plugs used tocontrol lost circulation situations.

A slurry according to the invention has a density of 0.9 g/cm³ to 1.3g/cm³, and is constituted by a solid fraction and a liquid fraction, hasa porosity (volume ratio of liquid fraction over solid fraction) of 38%to 50%, the solid fraction comprising:

60% to 90% (by volume) of lightweight particles having a mean size of 20microns (μm) to 350 μm;

10% to 30% (by volume) of micro-cement having a mean particle diameterof 0.5 μm to 5 μm;

0% to 20% (by volume) of Portland cement, having particles with a meandiameter of 20 μm to 50 μm;

0% to 30% (by volume) of gypsum; and

at least one alkali-resistant fibre present in an amount of less than 2%(by weight) and having a length of less than 6 cm and an aspect ratio ofgreater than 10.

The inclusion of the alkali-resistant fibres allows a high mechanicalstrength and improved resistance to erosion of set cement without theproblems of fibre degradation previously encountered in cement systems.

The term “fibre” used in relation to the present invention also includesribbon or platelet structures that accomplish the same performance asnormal fibre structures. Such materials may include a number ofdifferent materials: polymers, natural structures (ground plant fibres),but all have alkaline resistance, aspect ratio and size limits indicatedabove. The fibres can have various shapes, for example multi-lobed,curved, hooked, tapered, dumbb Where the fibre has a complex structure,only the exterior of the fibre needs to be alkaline resistant.Core-and-shell fibres can be used that are coated with an alkalineresistant material before use.

Preferably the cement slurry has porosity of less than 45%.

The lightweight particles typically have a density of less than 2 g/cm³,and preferably less than 0.8 g/cm³. These can be selected from hollowmicrospheres, in particular silico-aluminate microspheres orcenospheres, synthetic materials such as hollow glass beads, and moreparticularly beads of sodium-calcium-borosilicate glass, ceramicmicrospheres, e.g. of the silica-alumina type, or beads of plasticsmaterial such as polypropylene beads.

One or more additives, such as dispersants, antifreeze, water retainers,cement setting accelerators or retarders, and foam stabilizers can beadded to the slurry.

In one embodiment, the solid fraction of the slurry is preferablyconstituted by lightweight particles of diameter 100 μm to 350 μm and byparticles of micro-cement, the ratio of lightweight particles tomicro-cement being 70:30 to 85:15. The solid fraction of the mixture canalso be constituted by 50% to 60% (by volume) of first lightweightparticles having a mean diameter of 100 μm to 400 μm, by 30% to 45% ofsecond lightweight particles having a mean diameter of 20 μm to 40 μm,and by 5% to 20% of micro-cement.

The fibres used in the present invention preferably have a length of 2mm to 6 cm and diameters of 6 microns to 200 microns. The material fromwhich the fibre is made can vary but must be alkali-resistant andcompatible generally with aqueous cement slurries. Examples of suchfibres are high-zirconia fibres and phenolic polymer fibres.

A typical slurry falling within the scope of the invention can comprisesilico alumina microspheres (50 to 60% by volume of blend), microcement(10 to 20% by volume of blend) and fine silicoalumina microspheres (0 to30% by volume of blend). The fibres would be added to that slurry at aconcentration of 0.5 to 1.5 pounds of fibre per barrel of slurry. Thefibres are added to the slurry once the sl

Alternatives are:

-   -   Cenospheres: 55% by volume of blend    -   Finer cenospheres: 30% by volume of blend    -   Microcement: 15% by volume of blend    -   Fibres : 0.5 pounds per bbl

or:

-   -   Censopheres: 50% by volume of blend    -   Cement: 40% by volume of blend    -   Flyash: 10% by volume of blend.

The accompanying drawing shows a schematic view of an experimental testset-up used to test slurries according to the invention.

The test set-up has been created to test the efficiency of several fibretypes in plugging different size holes from 1 mm to 4 mm. The set-up isbased on a standard high temperature fluid loss cell 10, which is fittedwith an inside grid 12. The grid substitutes the usual filtration areaand it is placed at 20 mm above the bottom 14 of the cell as it isdepicted on the figure. The grid 12 is made with holes and threedifferent grids are available. The first grid is made with 100 holes of1 mm diameter which gives a total void space of 0.78 cm². The two othersare respectively with 100 holes of 2 mm diameter and with 25 holes of 4mm diameter, the both leads to a total void space of 3.14 cm². Thebottom cap 14 of the cell is open with a 1.1 cm diameter hole.

The test is run as a fluid loss test. 385 ml of slurry are placed insidethe cell above the grid. A metal plate 16 is placed on the top of theslurry. A pressure of 20 bars is applied for 30 minutes and the amountof slurry coming out through the grid is measured. The pressure can beapplied progressively or instantaneously from 0 to 20 bars. The pluggingefficiency is evaluated with the quantity of slurry collected after 30minutes, as well the speed of plugging the grid. The tests have beenperformed at room temperature.

This set-up allows to see the efficiency of the different fibers testedin plugging the different hole sizes, according to their type: material,shape and length, and to their concentration used in the slurry.

The efficacy and scope of the invention is demonstrated in the testsdescribed below.

Slurries

Two slurries are used to demonstrate the invention: a standard cementslurry having a density of 15.8 ppg (Std.), and a low-density slurryhaving a specific granulometric characterisation and density of 12.24ppg (LC) as described in WO 01/09056.

Fibres

Five types of fibres (A-E) are used as follows:

-   -   A: polyamide fibres around 18 μm in diameter, from 3 mm to 18 mm        long, with a density of 1 g/cm³;    -   B: fibrous glass fibres around 20 μm in diameter, from 10 mm to        14 mm long (average 12 mm), with a density of 2.55 g/cm³;    -   C: novoloid fibres around 21 μm in diameter, from 18 mm to 22 mm        long, with a density of 1.27 g/cm³;    -   D: polypropylene fibres around 0.7 μm in diameter, from 11 mm to        13 mm long, with a density of 0.9 g/cm³; and

E: polyester fibres around 13 μm in diameter, from 4 mm to 8 mm long.

The table below shows the results obtained using fibres A-E with Std.and LC slurries Grid Fibre Collected Collected Holes Concen- Slurry inml Slurry in ml Diameter Fibre tration (0 to 20 (0 to 20 (mm) SlurryType in kg/m3 bars prog) bars instant) 1 Std none — All All 1 Std C 9.717 — 1 Std B 19.2 125.5 — 1 Std D 13 130 — 1 LC none — All All 1 LC C9.7 4.4 — 1 LC B 19.2 4 — 1 LC D 13 10.2 — 2 Std A 6 mm 13 6.9 26.9 2Std A 12 mm 13 28.2 31.7 2 LC A 6 mm 13 2 2.5 2 LC A 12 mm 13 — 0.9 4Std A 12 mm 19.5 All All 4 Std A 18 mm 26 26.1 23 4 LC A 12 mm 16.5 0.2— 4 LC A 18 mm 16.5 0.8 1.3 6 LC A 18 mm 16.5 0.5 All

All of the fibres plug the 1 mm grid with the LC and Std slurries, butwith the LC slurry they are much more efficient. For larger holes 2 mmand 4 mm, the A fibres work well in plugging the holes and preventingthe slurry from passing through the grid, again, far more efficientlywith the LC slurry than with a Std slurry and at a lower concentrationof fibres.

Fibre E work as well in 1 mm and 2 mm grid but the efficiency is usuallylower than that of Fibre A

The efficiency of the A fibres increases with their concentration.

It is clear that the length of the fibres plays an important roleaccording to the size of the holes to be plugged. The longer fibres areable to plug larger hole. The 12 mm and 18 mm A fibres plug the 4 mmholes whereas the 6 mm A fibres does not.

The plugging of the holes is better when the fibres have a longer timeto bridge the holes, it is assessed by comparing the results when thepressure is applied progressively and instantaneously. In the case of Cfibres, there is no plugging when the pressure is appliedinstantaneously.

The combination of A fibres and a LC slurry is more efficient than witha standard slurry the plugging is faster and less slurry goes throughthe grid. Furthermore a lower fibre concentration can be used. Thisphenomenon is due to the particles size distribution of the LC slurryand the interaction between the slurry particles and the fibres.

1. A cement slurry having a density of 0.9 g/cm³ to 1.3 g/cm³, andcomprising a solid fraction and a liquid fraction with a porosity(volume ratio of liquid fraction over solid fraction)of 38% to 50%, thesolid fraction comprising: 60% to 90% (by volume) of lightweightparticles having a mean size of 20 microns (μm) to 350 μm; 10% to 30%(by volume) of micro-cement having a mean particle diameter of 0.5 μm to5 μm; 0% to 20% (by volume) of Portland cement, having particles with amean diameter of 20 μm to 50 μm; 0% to 30% (by volume) of gypsum; and atleast one alkali-resistant fibre present in an amount of less than 2%(by weight) and having a length of less than 6 cm and an aspect ratio ofgreater than
 10. 2. A system as claimed in claim 1, wherein the fibrescomprise glass having a high zirconia concentration or organic fibrescomposed of a phenolic polymer or other polymeric system.
 3. A cementslurry as claimed in claim 1 or 2, wherein the porosity is less than45%.
 4. A cement slurry as claimed in claim 1, 2 or 3, wherein thelightweight particles have a density of less than 2 g/cm³.
 5. A cementslurry as claimed in claim 4, wherein the lightweight particles have adensity of less than 0.8 g/cm³.
 6. A cement slurry as claimed in anypreceding claim, wherein the lightweight material is selected fromhollow microspheres, synthetic materials, ceramic microspheres, or beadsof plastics material.
 7. A slurry as claimed in any preceding claim,further comprising one or more additives including dispersants,antifreeze, water retainers, cement setting accelerators or retarders,and foam stabilizers.
 8. A slurry as claimed in any preceding claim,wherein the fibres have a length of 2 mm to 6 cm and diameters of 6microns to 200 microns.
 9. A slurry as claimed in any preceding claim,wherein the fibres are added to the slurry at a concentration of 0.5 to1.5 pounds of fibre per barrel of slurry
 10. A method of controllinglost circulation in a well being drilled, comprising placing, adjacent azone of lost circulation, a plug of a fibre-reinforced, low densitycement system for the control of lost circulation during the drilling ofsubterranean wells, generally comprising a cement slurry having adensity of 0.9 g/cm³ to 1.3 g/cm³, and comprising a solid fraction and aliquid fraction with a porosity (volume ratio of liquid fraction oversolid fraction) of 38% to 50%, the solid fraction comprising: 60% to 90%(by volume) of lightweight particles having a mean size of 20 microns(μm) to 350 μm; 10% to 30% (by volume) of micro-cement having a meanparticle diameter of 0.5 μm to 5 μm; 0% to 20% (by volume) of Portlandcement, having particles with a mean diameter of 20 μm to 50 μm; 0% to30% (by volume) of gypsum; and at least one alkali-resistant fibrepresent in an amount of less than 2% (by weight) and having a length ofless than 6 cm and an aspect ratio of greater than
 10. 11. A cementslurry having a density of 0.9 g/cm³ to 1.3 g/cm³, and comprising asolid fraction and a liquid fraction with a porosity (volume ratio ofliquid fraction over solid fraction)of 38% to 50%, the solid fractioncomprising: 60% to 90% (by volume) of lightweight particles having amean size of 20 microns (μm) to 350 μm; 10% to 30% (by volume) ofmicro-cement having a mean particle diameter of 0.5 μm to 5 μm; 0% to20% (by volume) of Portland cement, having particles with a meandiameter of 20 μm to 50 μm; 0% to 30% (by volume) of gypsum; and atleast one alkali-resistant fibre present in an amount of less than 2%(by weight) and having a length of less than 6 cm and an aspect ratio ofgreater than
 10. 12. A system as claimed in claim 11, wherein the fibrescomprise glass having a high zirconia concentration or organic fibrescomposed of a phenolic polymer or other polymeric system.
 13. A cementslurry as claimed in claim 11, wherein the porosity is less than 45%.14. A cement slurry as claimed in claim 11, wherein the lightweightparticles have a density of less than 2 g/cm³.
 15. A cement slurry asclaimed in claim 12, wherein the porosity is less than 45%.
 16. A cementslurry as claimed in claim 15, wherein the lightweight particles have adensity of less than 2 g/cm³.
 17. A cement slurry as claimed in claim11, wherein the lightweight particles have a density of less than 0.8g/cm³.
 18. A cement slurry as claimed in claim 11, wherein thelightweight material is selected from hollow microspheres, syntheticmaterials, ceramic microspheres, or beads of plastics material.
 19. Aslurry as claimed in claim 11, further comprising one or more additivesincluding dispersants, antifreeze, water retainers, cement settingaccelerators or retarders, and foam stabilizers.
 20. A slurry as claimedin claim 11, wherein the fibres have a length of 2 mm to 6 cm anddiameters of 6 microns to 200 microns.
 21. A slurry as claimed in claim11, wherein the fibres are added to the slurry at a concentration of 0.5to 1.5 pounds of fibre per barrel of slurry.
 22. A cement slurry havinga density of 0.9 g/cm³ to 1.3 g/cm³, and comprising a solid fraction anda liquid fraction with a porosity (volume ratio of liquid fraction oversolid fraction)of 38% to 50%, the solid fraction comprising: 60% to 90%(by volume) of lightweight particles having a mean size of 20 microns(Jum) to 350 μm; 10% to 30% (by volume) of micro-cement having a meanparticle diameter of 0.5 μm to 5 μm; 0% to 20% (by volume) of Portlandcement, having particles with a mean diameter of 20 μm to 50 μm; 0% to30% (by volume) of gypsum; and at least one alkali-resistant fibrepresent in an amount of less than 2% (by weight) and having a length ofless than 6 cm and an aspect ratio of greater than 10, wherein thefibres comprise glass having a high zirconia concentration or organicfibres composed of a phenolic polymer or other polymeric system.
 23. Acement slurry as claimed in claim 22, wherein the porosity is less than45%.
 24. A cement slurry as claimed in claim 22, wherein the lightweightparticles have a density of less than 2 g/cm³.
 25. A cement slurry asclaimed in claim 22, wherein the lightweight particles have a density ofless than 0.8 g/cm³.
 26. A cement slurry as claimed in claim 22, whereinthe lightweight material is selected from hollow microspheres, syntheticmaterials, ceramic microspheres, or beads of plastics material.
 27. Aslurry as claimed in claim 22, further comprising one or more additivesincluding dispersants, antifreeze, water retainers, cement settingaccelerators or retarders, and foam stabilizers.
 28. A slurry as claimedin claim 22, wherein the fibres have a length of 2 mm to 6 cm anddiameters of 6 microns to 200 microns.
 29. A slurry as claimed in claim22, wherein the fibres are added to the slurry at a concentration of 0.5to 1.5 pounds of fibre per barrel of slurry.
 30. A method of controllinglost circulation in a well being drilled, comprising placing, adjacent azone of lost circulation, a plug of a fibre-reinforced, low densitycement system for the control of lost circulation during the drilling ofsubterranean wells, generally comprising a cement slurry having adensity of 0.9 g/cm³ to 1.3 g/cm³, and comprising a solid fraction and aliquid fraction with a porosity (volume ratio of liquid fraction oversolid fraction)of 38% to 50%, the solid fraction comprising: 60% to 90%(by volume) of lightweight particles having a mean size of 20 microns(μm) to 350 μm; 10% to 30% (by volume) of micro-cement having a meanparticle diameter of 0.5 μm to 5 μm; 0% to 20% (by volume) of Portlandcement, having particles with a mean diameter of 20 μm to 50 μm; 0% to30% (by volume) of gypsum; and at least one alkali-resistant fibrepresent in an amount of less than 2% (by weight) and having a length ofless than 6 cm and an aspect ratio of greater than 10.